A resettable, thermally sensitive over-current protector element is an electrical device that conducts current in a low resistance state until the current reaches a sufficiently high level, at which time it will drastically change to a high resistance state. This serves to, in effect, choke off the current, thereby offering over-current protection to the source or destination of the current. The device is said to be thermally sensitive in that the transition between low resistance and high resistance occurs when the device is sufficiently heated (due to the increased current). Resettable means that the protector can automatically return to its low resistance state once current through it has dropped to a sufficiently low level and the device has cooled down. An example of such a device is a conductive polymer positive temperature coefficient (PTC) element that drastically changes from a low resistance state to a high resistance state in response to sufficiently high current passing through it. This change in resistance state may be, for example, at least three orders of magnitude.
PTC elements are often used within rechargeable battery packs in order to restrict the battery current, to thereby help protect the battery from an over-current condition. Once the protection specification for the battery has been determined, e.g. the battery should not be allowed to source more than 5 A continuously for five seconds, a suitable PTC element is selected, so that the transition between the low resistance and high resistance states (the transition temperature) corresponds to the desired battery protection specification. Unfortunately, a PTC element by itself is not accurate enough in many instances, in part due to manufacturing process variations. For instance, consider a desired scheme where the battery current should be cut off when it has reached 5 A for five seconds continuously; in practice, the PTC element may not be able to transition into its high resistance state until a substantially longer time interval has elapsed. In other instances, manufacturing variations may cause the PTC element to trip too early. While efforts have been made to more precisely control when an over-current protector is tripped, those techniques are unable to meet more stringent accuracy requirements.